Process for treatment of water injection wells

ABSTRACT

Water injection wells are stimulated by an oil-external micellar slug preceded and/or followed by at least one micellar (buffer) slug of a higher water content. This series of slugs is forced out into the formation by means of a water drive. There may be continuous grading from water to oil-external slug to drive water. The buffer slugs may be continuously graded in water content rather than by injecting distinctly different slug compositions.

United States Patent Dauben et al. [4 1 June 20, 1972 [541 PROCESS FORTREATMENT OF 3,254,714 6/1966 Gogarty et al. ..166/274 WATER INJECTIONWELLS 3,343,597 9/1967 Gogarty et al. 3,4 7,18 1 6 t 72 Inventors:Dwight L. Dauben; 11. 11. ironing; Loyd 3 2., 3 3 g fi a] W. Jones, allof Tulsa Okla. 08 y 3,557,873 1/ 1971 Owens 166/274 [73] Assignee: AmocoProduction Company Primary Examiner-Stephen J. Novosad [22] Ffled' May1,70 Attomey-Paul F Hawley and Arthur Mcllroy [2]] Appl. No.: 38,365

[57] ABSTRACT U-S- Cl. R, water injection wells are stimulated aniLextemal mice]- 1521b 43/25 lar slug preceded and/or followed by atleast one micellar [58] Field of Search 166/273, 274, 275, 305, 304(bufi'er) slug of a higher water content. This series f slugs is forcedout into the formation by means of a water drive. There [56] defencesCmd may be continuous grading from water to oiI-extemal slug to P drivewater. The buffer slugs may be continuously graded in water contentrather than by injecting distinctly different slug 3,412,791 11/1968Gogarty 166 273 composition, 3,476,184 11/1969 Davis, Jr ....166/2733,275,075 9/1966 Gogarty et al. ..l66/274 13 Claims, 1 Drawing FigureINJECTION PRODUCTION WELL WELL 5T.- v I Q i [I BUFFER BUFFER SLUG(S)swets) I I 7 I l 111115011011 commuous GRADING PR'MARY 1 l I WATER OFWATER :JNTENT MICELLAR O'L I 1 i 0111-: OR MORE SLUGS SLUG I I I EACHCONTAINING A I I FIXED BUT DIFFERENT I I AMOUNT OF WATER I I i lCONTINUOUS GRADING OF WATER CONTENT 0R ONE OR MORE SLUGS EACH CONTAININGA FIXED BUT DIFFERENT AMOUNT OF WATER PROCESS FOR TREATMENT OF WATERINJECTION WELLS INTRODUCTION Our invention relates to the use ofmicellar solutions to improve the injectivity of water injection wellsemployed in oil recovery operations. It is particularly concerned withan improvement and novel method of treating such wells whereby thetendency of said solutions to form troublesome emulsions is held to aminimum, thus making possible the treatment of injection wellspenetrating extremely low permeability (e.g., less than 20 md.)formations.

BACKGROUND OF THE INVENTION The injectivity of water injection wells canbe improved by utilizing a process capable of removing residual oilsaturation and/or organic deposits from around the well bore.Restrictions in water injectivity occur in formations in which thepermeability to water at residual oil saturation is a small percentageof the permeability at 100 percent water saturation. These percentagesnormally range from 2-15 percent for water-wet formations, 15-30 percentfor intermediate-wet formations and 30-80 percent for oil-wetformations. Based upon radial flow calculations, two-fold injectivityimprovements can be obtained by removing residual oil saturation out toabout 45 feet in reservoirs where permeability to water is not more thanabout percent of the absolute permeability.

Organic deposits, such as paraffins and asphaltenes, are present in someinjection wells. Such deposits are usually never completely removed ifthe well was converted from a producer to an injection well. Inaddition, the initial injection of relatively cold water into the matrixaround the well bore can result in paraffin precipitation in somereservoirs. If the well also contains a fracture or other stimulatedcondition, the presence of such deposits may not show up as a skineffect from pressure fall-off tests. The magnitude of injectivityimprovements achieved by dissolving and dispersing organic deposits can,depending upon extent of the damage, lead to several-fold increases ininjectivity.

The use of micellar solutions as a solvent or agent in injection wellcleanout procedures has been known for some time but was practiced withindifferent success. More recently, the use of oiland water-extemalmicellar systems has been described in U.S. Pat. Nos. 3,467,188 and3,474,865, respectively. Water-external systems naturally have lowersolvent power for organic deposits such as paraflin and asphaltenenormally found at or near the face of an injection well, and also arepoor solvents for the trapped oil frequently present near the face ofthe injection well.

Provided that the rock permeability level is sufficiently high, a singlemicellar slug followed by water can be used to improve injectivity,i.e., the emulsion plugging efiect, resulting from contact of the slugwith the drive water, generally is tolerable. For low permeability rockscharacteristic of the West Texas carbonate reservoirs, plugging of rockpores can occur, particularly in formations having permeabilities of theorder of from about 1 to about 3 md., Field Results of Injection WellStimulation Treatments Using Micellar Dispersions, by J. P. Moran etal., Soc. Pet. Eng. of AIME, paper No. 2842. Such plugging apparentlyresults when the single solvent slug contacts a large amount of water atthe front end and when driving water rapidly mixes with the slug at itstrailing edge. The water, being much more mobile than the slug, fingersthrough it and creates zones of high water saturation which in turnleads to the formation of plugging emulsions by excessive dilution ofthe single solvent slug. This latter efiect could be reduced to someextent by driving with water containing mobility reducing polymers, butsuch practice is costly and the polymers themselves have a tendency toplug the rock.

DESCRIPTION OF THE INVENTION We have found that the aforesaid waterinjection wells can be stimulated successfully by the use of a slug ofan oil-externa] micellar solution wherein such solution is precededand/or followed by at least one micellar slug of a higher water contentand then followed finally by injection water as the drive agent. Alsoincluded within the scope of this invention is the practice ofcontinuously grading from water to oil-extemal slug to drive water. Theoil-extemal solution mentioned above is referred to herein as the"primary" slug and usually contains from about 5 to about 60 percentwater. The other compositions are referred to herein as butfer" slugsand may be applied by continuously grading the water content or byinjecting distinct slug compositions. Improvements can also be obtainedwithout use of a micellar buffer slug preceding the primary slug butemploying at least one micellar solution of higher water content at thetrailing edge of the solvent or primary slug. Generally, however, weprefer to use the high water content micellar solution at the front ofthe primary slug because contamination of the latter by in-place wateris materially reduced, in turn reducing the possibility of undesirableemulsion formationQThe use of such micellar solutions at the leadingedge of the primary slug is desirable to prevent contamination of theslug when it is initially injected into the sand face. The desirabilityof these graded micellar solutions at the trailing edge of the primaryslug is much greater, however, for at least two reasons. First, thegraded solution at the primary slugs trailing edge protects the slugfrom contamination by the drive water and subsequent emulsion formationin the slug, thus maintaining relatively easy flow of the slug materialthrough the formation. Being micellar solutions, the relatively highwater content solutions, e.g., 70-90 percent, have a higher viscositythan the drive water, making them a more effective means to push theprimary slug through the formation without causing bypassing of thedrive water through and around the slug itself.

The non-aqueous portion of the micellar solution is referred to as theconcentrate which consists essentially of a hydrocarbon, a surfactantand a cosurfactant. The concentrate may be modified by varying theratios of ingredients therein and in some cases may be free of addedhydrocarbons. The amount of micellar solution, including primary andbuffer slugs, injected into the zone to be treated amounts to from about0.5 to about 20 barrels per vertical foot of sand, corresponding to fromabout 0.2 to about 10 bbls of the concentrate.

The primary slug in order to obtain maximum efiectiveness as aninjectivity irnprover under a wide variety of reservoir conditionsshould possess the following characteristics. It should be oi] externalso that it-will readily contact the organic phase and have thecapability both of dissolving organic deposits and removing residual oilsaturation. The slug may incorporate solvents such as kerosene oraromatic naphtha. Where asphaltene deposits are known to exist, asolvent of high aromatic content is preferred because of its increasedability to dissolve deposits of this kind. As an alternate procedure,small amounts of an additional solvent, such as ketones or carbondisulfide, may be added to the kerosene or equivalent to increase theability of the micellar solution to dissolve or solubilize saiddeposits. Peptizing or dispersing agents may also be added. Byincorporating these solvents in relatively small amounts, the usualproblems of high vapor pressure and fire hazard are reduced.

To eflectively increase injectivity the oil-extemal micellar slug shouldnot only be miscible with oil but also should not be trapped by thedrive water. Since the slug itself contains a high hydrocarbon content,trapping of the slug can leave a permanent oil saturation. In addition,if water is indiscriminately mixed with an oil-extemal micellar slug, ablocking emulsion may be formed. Such an adverse condition can occur inthe rock due to fingering of water through the slug. in low permeabilityformations this can cause complete blocking. Experience has shown thatwhile it is possible to design a micellar slug composition to minimizeemulsification, it has not been possible to produce a composition thatwill resist emulsion formation entirely when indiscriminately mixed withlarge volumes of water. Formation plugging emulsions are avoided in thepresent invention by the use of the buffer slugs referred to above,which prevent direct dilution of the primary slug by large proportionsof water.

Based on experimental and field data, we have observed that plugging ofthe formation immediately adjacent to the injection well can be avoidedby the introduction of suitable buffer slugs to prevent the direct andrapid contact of the primary slug with connate and drive water. Thebuffer slug composition may be either a water diluted form of theprimary slug, or essentially the equivalent thereof, or a speciallyblended high water content solution. For example, as the hydrocarbon tosurfactant ratio is reduced, the solution will grade more smoothly towater, i.e., will be more emulsion free. Special blends may be preparedfrom concentrates having a low ratio of oil to chemical (surfactant andcosurfactant). The ratio of surfactant to cosurfactant may also bevaried to achieve desired properties. Thus, for example, as the ratio ofcosurfactant to surfactant is increased the system's resistance toemulsification increases. For this reason the cosurfactant to surfactantratio in both the leading and trailing buffer slugs generally should behigher than that in the primary slug. With the highercosurfactant-surfactant ratio the bufier slugs on contact with eitherformation water or drive water are more resistant to emulsion formation.The tendency of such systems to resist emulsion when thecosurfactant-surfactant ratio is increased is clearly demonstrated bythe ternary diagrams shown in FIGS. 1 and 3 of U.S. Pat. No. 3,308,068.

The buffer slug may be prepared and injected batchwise, with each batchvarying in water content. Alternatively, water may be added continuouslyduring injection to achieve a smooth, controlled gradation in watercontent.

The micellar solution concentrate from which the primary and bufferslugs may be prepared is composed of a hydrocarbon solvent, a surfactantand a cosurfactant. As typical examples of these three components theremay be mentioned, respectively, kerosene, a hydrocarbon sulfonate, and amixture of an ethoxylated aliphatic alcohol having from four to carbonatoms and a C alcohol or fusel oil, the volume ratio of ethoxylatedalcohol to C alcohol or fusel oil ranging from about 2:1 to about 7: 1.Fresh water with added sodium chloride or equivalent salt is blended atthe well site to form the final micellar solution mixtures. Thesolutions are miscible with most reservoir crude oils, with fresh water,and, in general, are stable at temperatures ranging from 32 F. to about200 F. One composition may thus be used in many different reservoirapplications, the main consideration being the salinity of the availablewater supply. These solutions may be used in reservoirs of anypermeability level but the proces of our invention is especiallyapplicable to the very low permeability reservoirs, i.e., under 20 md.For low permeability applications the process of our invention typicallyemploys the following sequence of compositions and injection steps:

Step No.

Inject: 1 Highest water content buffer slug (70-95% water) Intermediatewater content buffer slug (40-80% water) Oil-extemal micellar slug(5-60% water) Intermediate water content buffer slug (40-80% water)Highest water content buffer slug (70-95% water) Water Injection murewwwhere organic deposits are a known problem, a larger than normal volumeof the primary slug may be injected for maximum benefits. Where it isdesired to remove oil saturation from the largest possible area aroundthe well bore, larger than normal volumes of the buffer slugs may beemployed.

Both the amount and type of salt present in the primary slug have asubstantial influence on its properties. Transparent solutions areformed within a certain limit of salinity in the water. At low saltlevels, typically under 5,000 ppm, solutions are often clear butviscous, hazy or emulsified. The preferred transparent solutionstypically occur in salt concentration ranges of from about 5,000 toabout 20,000 ppm. The desired salt level of water in buffer slugs isless than for the primary slug. The experimental data indicate that thesalt level required to achieve transparent high water solutionsdecreases linearly as the water content is increased.

The process of our invention may be very readily applied in the field.The non-aqueous components, i.e., hydrocarbon solvents, sulfonate, andcosurfactant, may be blended at a convenient location, such as at arefinery. The mixture may then be transported to the well which is to betreated and water in the correct proportion is then blended to form thedesired primary slug composition. Water is then added and mixed in withthe primary slug or to a part of the non-aqueous mixture in an amount toform the desired buffer slug composition. Solutions should then befiltered and injected into the reservoir in the proper sequence.

In some cases where the salt content of the in-place water is greaterthan about 5,000 ppm, we have found it desirable to inject initially aslug of fresh water (such as potable water), usually from about 0.5 to 5barrels per vertical foot of formation to be treated. While the lattersalt content of the injection water applies to formations having apermeability not greater than about 20 md., formations havingpermeabilities of at least md. can tolerate the use of waters, the saltcontent of which may be as high as 30,000 ppm, before the use of freshwater slugs, as described herein, are generally needed. A preflush isparticularly desirable to displace waters containing several hundred ppmof divalent cations. This procedure aids in protecting the leadingbuffer solution(s) as well as the primary slug from the possibility ofsalt contamination and subsequent emulsification which in turn causesformation plugging. The leading buffer solution can withstand high saltconcentrations much better than the primary slug which containsappreciably more of the micellar concentrate. With the presence of thefresh water bank ahead of the buffer solution, however, the entiresystem of micellar solutions is protected from emulsification and thesubsequent plugging of the formation. Likewise, it may be desirable toplace a slug of fresh water back of the most dilute trailing buffer slugand just in front of the injection water where the latter has anobjectionably high salt content. In cases where water-sensitiveformations are involved, we prefer to initially comrningle fresh waterwith a relatively small amount of injection water, followed by a slug offresh water.

Although only a single micellar slug injection may be adequate for thehigher permeability level rocks, certain advantages in cost oreffectiveness can be derived by employing one or more bufi 'er sings inaccordance with our invention. The buffer slugs, although not veryeffective in dissolving organic deposits, are generally effective inremoving residual oil saturation and avoid entirely the trapping of thenricellar slug. Buffer slugs, because of their much higher watercontent, are cheaper than the primary slug. Consequently, for a givenfluid cost, a greater volume of the reservoir can be effectivelycontacted. Conversely, for a given volume of rock swept, the amount andcost of slug materials required are reduced.

While oil-external micellar solutions generally known to the art may beused in carrying out the process of our invention, micellar solutionsconsidered particularly applicable are those described in US. Pat. Nos.3,126,952 and 3,308,068 and in copending U.S. Ser. No. 848,682, filedAug. 8, 1969 by Loyd W. Jones et al. Typical of the compositions taughttherein comprise a C to C hydrocarbon mineral oil, a cosurfactant suchas diethylene glycol mono-hexyl ether and a surfactant such as apetroleum mahogany alkali metal sulfonate, in which the weight ratio ofthe sulfonate to the mono-hexyl ether representative brine solution2,000 ppm total salts). In the case of the core subjected to the processof our invention, two buffer slugs of the indicated composition wereused before and after the primary slug composition. In this case thebuffer ranges from about 7:3 to about 1:9. Another type of primary lugwere prepared from the primary slug.

Initial Permeabilities, rnd. saturations Run I Drivin Resid- N 0. CoreOil Brine Primary slug composition Buffer slugs brine Initial ual oilFinal 1 Fir d Level- 1,000 18.92% petroleum hydrocarbon sodiumsullonate, 28.38% None 1,000 347 53 273 berea. land. p.p.m kerosene,47.33% Water plus 1% NaCl, 5.3% *Cosurlacp.p.m

salt. tant. salt. 2 V h Lev l- 1,000 19.25% petroleum hydrocarbon sodiumsulfonate, 28.87% N one 1,000 2, 4 .329 353 land. p.p.m kerosene, 48.12%water plus 1% NaCl, 3.75% "Cosurp.p.m

salt. iactant. Salt. 3 Level- Level- 1,000 16.99% petroleum hydrocarbonsodium sulfonate, 31.55% 80% 11 0, 1,000 4.6 1. 35 3. 7

land. land. p.p.m. kerosene. primary slug. p.p.m.

salt. 48i54g/i, N NaCl plus 292 p.p.m. CaCl. 2.92% *Cosur- 40% H70, 60%salt.

ac an primary slug.

*0 mol ethylene oxide adduct 01' l-hexanol plus isoamyl alcohol, molratio 1:1. Same as in Run No. 1 except the ratio of 6 mol adduct toisoamyl alcohol was 3.5;1.

"*6 mol adduct plus fusel oil in ratio of 1.84:1.

slug may typically consist essentially of an alkali metal petroleumsulfonate or a synthetic hydrocarbon sulfonate having an averagemolecular weight of from about 350 to about 600, a cosurfactantconsisting of from 2-12 mols of an ethylene oxide adduct of a primaryalcohol having from four to l0 carbon atoms, the ratio of the sulfonateto cosurfactant ranging from about 3:1 to about 6:1, the combination ofsulfonate and cosurfactant being present in a combination ranging fromabout 8 to about 30 weight percent; from about 2 to about 20 weightpercent of a hydrocarbon oil, e.g., kerosene, and not more than about 95weight percent water which may range in salinity from that of distilledwater up to about 20,000 ppm of a salt of a monovalent cation.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION In theaccompanying drawing a bufier slug is injected into the input well toestablish a bank 2 next to formation oil bank 4. The leading edge ofbank 2 contains the highest concentration of water. This fluid isgenerally capable of removing residual oil saturation but is relativelyineffective in removing organic skins in the vicinity of the well bore.The interfacial tension between the oil and bank 2 is very low, e.g.,about 0.01 dyne/cm. The water content of bank 2 decreases as the primarymicellar slug 6 is approached. This change in water content, aspreviously mentioned, may be either gradual or in the form of a seriesof distinct slugs of fixed but different concentration increasing inamount of the non-aqueous phase in the direction of the primary micellarslug 6. Sufficient concentrated (50 percent non-aqueous phase) micellarsolution is introduced via the injection well to form the primarysolvent slug 6. Thereafter, a second buffer slug is added through theinjection well, usually in an amount corresponding to from about 2 to 4times the volume of bank 2, resulting in the formation of buffer bank 8.Following the latter, injection water 10 is introduced, forcing theaforesaid slugs through the formation in banks which tend to dissipateafter they have moved out from the injection well a substantialdistance, e.g., to feet. Under these conditions any organic deposits onthe well bore face as well as residual oil saturation in the vicinity ofthe well bore are dissolved and carried in dilute form out in theformation. Water injection can then be continued at an injection ratesubstantially increased over that possible prior to treatment.

The process of our invention will be further illustrated by the exampleswhich follow. The data appearing herein are based on both laboratory andfield work. In the laboratory investigation the formation core undertest was first saturated with water, then with oil to establish an oilsaturation. Thereafter, the core was subjected to waterflooding with aAlthough the slug compositions employed in the above example variedslightly from one another, such minor differences are not considered toafi'ect the conclusions made from the data obtained. Test results showthat in the higher permeability Berea core (347 md initial permeability)a single slug driven by water was successful in restoring percent of theinitial permeability. Although we did not employ our invention intreatment of the same kind of core, we anticipate that the use of bufierslugs as taught herein would lead to further improvement in thispermeability level rock. In the lower permeability rocks whereinjectivity problems are most severe, the advantage of the use of bufferslugs is demonstrated. Thus, in addition to the tests cited in the aboveTable, i.e., Run No. 2, plugging or lack of any improvement was observedin five other low permeability rock tests utilizing a single oil-extemalmicellar solution. Run No. 3 demonstrates the improvement in lowpenneability rock achieved by the process of our invention in which theoil saturated core was restored to approximately 80 percent of itsinitial permeability.

In the field operations the micellar concentrate was delivered andstored in a clean 300-barrel tank. Fresh water was brought to the welllocation in clean acid transport trucks. Preparation of the threemicellar solutions, i.e., the primary slug and two buffer slugs ofvarying water content, was carried out in a tank having two 250-barrelcompartments and in a 96- barrel acid transport tank. The mixingprocedure consisted of initially placing a quantity of fresh water inthe proper tank, adding and circulating into solution a prescribedquantity of sodium chloride and finally blending in the required amountof the micellar concentrate. All fluids were filtered before use.

EXAMPLE I 27.9 weight percent petroleum hydrocarbon sodium sulfonate(molecular weight about 440) 7 weight percent cosurfactant made up ofthe 6 mol ethylene oxide adduct of l-hexanol, and fuse] oil; these twocomponents of the cosurfactant being present in a volume ratio of 6:1

From the above composition the primary slug was prepared by mixing 45bbls of the concentrate with 45 bbls of fresh water (Ogallala water)containing 12,000 ppm of added sodium chloride. The leading bufier slugpercent water) was prepared from 8 bbls of the concentrate and 72 bblsof fresh water containing 4,000 ppm of added sodium chloride. The secondbuffer slug (70 percent water) was formulated from 15 bbls of theconcentrate and 35 bbls of fresh water containing 8,000 ppm of addedsodium chloride.

After these solutions were prepared they were injected in a volumecorresponding to l A bbls of concentrate per vertical foot of formationinto the well in accordance with the following sequence and in thevolumes stated. Afler this the following injection procedure wasemployed:

l. Injected 15 bbls of Bufier Slug I (90 percent water) 2. Injected l5bbls of Buffer Slug II (70 percent water) 3. Injected 90 bbls of primarymicellar slug (50 percent water) 4. Injected 35 bbls of Buffer Slug II(70 percent water) 5. Injected 65 bbls of Bufier Slug I (90 percentwater) 6. Resumed normal injection Following treatment the waterinjectivity capacity was observed to increase to 505 barrels per day at1,000 psi. More than three months after treatment the well stillretained this same increase in injection rate, representing a gain ofabout l4 percent over the before-treatment injection capacity.

EXAMPLE II A well in the Northwest Mallet Unit of the Slaughter Field,Texas, having an initial injection capacity of I barrels per day at1,300 psi was treated with micellar solutions of the same compositionand in accordance with the same procedure as described in Example I. Thevolumes of micellar solutions used were the same as those shown inExample I, differing only in salt content. From the concentrate, theprimary slug (50 percent water) was prepared by mixing 45 bbls of theconcentrate with 45 bbls of fresh water containing 13,000 ppm addedsodium chloride. The leading bufier slug (90 percent water) wasformulated by mixing 8 barrels of concentrate with 72 barrels of freshwater containing 3,500 ppm sodium chloride. The second buffer slug (70percent water) consisted of 15 bbls of the concentrate mixed with 35bbls of fresh water containing 7,500 ppm added sodium chloride.

After these solutions were prepared they were injected in a volumecorresponding to l l: bbls of concentrate per vertical foot of formationinto the well in accordance with the following sequence and in thevolumes stated: I

I. Injected 100 bbls of fresh water into the zone to be .treated for aperiod of 24 hours to protect the micellar solutions from the highlysaline water already injected into the formation and which was to beinjected after treatment.

2. Injected l bbls of Buffer Slug I 90 percent water) 3. Injected l5bbls of Bufier Slug ll 70 percent water) 4. Injected 90 bbls ofprimarymicellar slug (50 percent water) 5. Injected 35 bbls of BufferSlug II 70 percent water) 6. Injected 65 bbls of Buffer Slug 1 (90percent water) 7. Injected 100 bbls of fresh water over a 24-hour period8. Resumed normal injection Following treatment the water injectivitycapacity was observed to increase to 125-130 barrels per day at 1,340psi. More than 3 months following treatment the well still retained thissame increase in injection rate, representing an increase of about 25-30percent of the before-treatment injection capacity.

EXAMPLE III A second well in the Levelland Unit of the Levelland Field,

Texas, having an initial injection capacity of 650 barrels per PrimarySlug: Mixed 140 bbls of concentrate with I40 bbls fresh water containingsuflicient sodium chloride to give a salt concentration of [2,000 ppm.

chloride to give a salt concentration of 8,000 ppm.

Buffer Slug No. I:

Buffer Slug No. II:

After these solutions were prepared they were injected (3 barrels pervertical foot) into the formation being treated in accordance with thefollowing sequence and in the volumes stated.

I. Injected 20 bbls of Bufler Slug I 2. Injected 20 bbls of Buffer SlugII 3. Injected 280 bbls of the primary slug 4. Injected I 37 bbls ofBufl'er Slug II 5. Injected 210 bbls of Buffer Slug I 6. Resumed freshwater injection Immediately following treatment the water injectivitycapacity was observed to increase to 710 barrels per day at 1,000 psi.More than 3 months after treatment the well still retained this sameincrease, representing an improvement of about ll percent over thebefore-treatment injection capaci- EXAMPLE IV This example illustratesthe difference in results obtained when a water injection well istreated with a primary slug without the benefit of buffer slugs andwells treated in accordance with the process of the present invention.This well was located in the Levelland Unit of the Levelland Field,Texas, and had an initial water injection capacity of 390 barrels perday at 855 psi. The micellar solution employed as the primary slug had acomposition comparable to that described in U.S. Pat. No. 3,467,188. Themicellar solution was prepared from bbls of the concentrate and I40 bblsof fresh water. The resulting slug was then injected into the formationbeing treated in an amount corresponding to 2.4 bbls of concentrate pervertical foot of oil-bearing formation. The micellar solution was thenforced out into the formation by injection of fresh water. Followingtreatment thewater injectivity capacity was first shown to be 330barrels per day at 840 psi, and approximately 1 month thereafter hadincreased to 400 barrels of water per day at 835 psi, representing anincrease of about 2.5 percent over the before-treatment injection level.

In all of the above examples the matrix permeability was less than 5 md,and with the oil saturation present the actual permeability to water wasnot more than I md.

We claim:

1. In the injection of a water stream through an injection well into aporous subsurface formation containing a water immiscible phase which isresistant to displacement by water flow and which substantiallyrestricts the flow of water through the formation, the improvement whichcomprises:

removing said water immiscible phase and thereby increasing theinjectivity of said formation to water with a resulting substantialincrease in the rate of water injection by injecting into the formationa micellar slug (1]) followed by injection into said formation of aprimary oil-extemal micellar solution in an amount suficient to displacesaid water immiscible phase out into said formation, said micellar slug(II) having a water content greater than that of said primaryoil-extemal solution;

following the injection of said oil-external solution with a micellarslug (1) of substantially increased water content,

thereby displacing said water immiscible phase by said oil-extemalsolution into the region about the well bore; thereafter injecting waterinto said well and into said formation in suflicient amount to displacesaid oil-external continuing said water injection at a substantiallyincreased rate.

2. The process of claim 1 wherein micellar slug (II) is graded inconcentration, being most dilute with respect to the non-aqueous phasethereof at the leading edge of slug (ll).

3. The process of claim 2 wherein micellar slug (I) at the trailing edgeof said primary oil-extemal solution is graded in concentration, beingmost dilute with respect to the non-aqueous phase thereof at thetrailing edge of micellar slug (I).

4. The process of claim 3 wherein the total volume of the non-aqueousphase employed in said primary solution and in slugs (I) and (II) andinjected into said fonnation amounts to from about 0.2 to about 10barrels per vertical foot of formation treated.

5. The process of claim 1 wherein a slug of fresh water in an amountsufficient to form a bank thereof is injected into said formation beforemicellar slug (II) and after micellar slug (I).

6. The process of claim 1 wherein said primary oil-external solutioncontains from about 5 to about 60 percent water and slugs (I) and (ll)contain more than 60 percent but not more than about 95 percent water,slugs (I) and (II) being most dilute with respect to the non-aqueousphase thereof at the trailing and leading edges, respectively.

7. The process of claim 1 wherein the total volume of the non-aqueousphase employed in said primary solution and in slugs (I) and (II) andinjected into said formation amounts to from about 0.2 to about barrelsper vertical foot of formation treated.

8. The process of claim 1 wherein slugs (l) and (II) are injectedbatchwise into the formation.

9. The process of claim 1 wherein water is added continuously to produceslugs (l) and (II) of variable water contents during their injectioninto the formation.

10. The process of claim 1 wherein slugs (I) and (II) have a lowerhydrocarbon to surfactant ratio than said primary oilexternal solution.

11. The process of claim 1 wherein the ratio of the total weight ofinorganic salts to the weight of total water in slugs (I) and (H islower than in the oil-external solution.

12. The process of claim 1 wherein the cosurfactant to surfactant ratioin slugs (I) and (II) is greater than that in the primary oil-externalsolution.

13. In the injection of a water stream through an injection well into aporous subsurface formation containing a water immiscible phase which isresistant to displacement by water flow which substantially restrictsthe flow of water through the formation and wherein the inplace water insaid formation has a salt content in excess of about 5,000 ppm, theimprovement which comprises:

removing said water immiscible phase and thereby increasing theinjectivity of said formation to water with a resulting substantialincrease in the rate of water injection by first injecting a slug offresh water into said formation;

thereafter injecting into the formation a primary oil-external micellarsolution in an amount sufficient to displace said water immiscible phaseout into said formation;

following the injection of said oil-external solution with a micellarslug (1) of substantially'increased water content over said oil-externalsolution, thereby displacing said water immiscible phase by saidoil-external solution into the region about the well bore;

thereafter injecting water into said well and into said formation insufficient amount to displace said oil-external micellar solution andsaid micellar slug (1) farther out into said formation and saturatingsaid region in turn with water; and

continuing said water injection at a substantially increased rate.

I i t i

2. The process of claim 1 wherein micellar slug (II) is graded inconcentration, being most dilute with respect to the non-aqueous phasethereof at the leading edge of slug (II).
 3. The process of claim 2wherein micellar slug (I) at the trailing edge of said primaryoil-external solution is graded in concentration, being most dilute withrespect to the non-aqueous phase thereof at the trailing edge ofmicellar slug (I).
 4. The process of claim 3 wherein the total volume ofthe non-aqueous phase employed in said primary solution and in slugs (I)and (II) and injected into said formation amounts to from about 0.2 toabout 10 barrels per vertical foot of formation treated.
 5. The processof claim 1 wherein a slug of fresh water in an amount sufficient to forma bank thereof is injected into said formation before micellar slug (II)and after micellar slug (I).
 6. The process of claim 1 wherein saidprimary oil-external solution contains from about 5 to about 60 percentwater and slugs (I) and (II) contain more than 60 percent but not morethan about 95 percent water, slugs (I) and (II) being most dilute withrespect to the non-aqueous phase thereof at the trailing and leadingedges, respectively.
 7. The process of claim 1 wherein the total volumeof the non-aqueous phase employed in said primary solution and in slugs(I) and (II) and injected into said formation amounts to from about 0.2to about 10 barrels per vertical foot of formation treated.
 8. Theprocess of claim 1 wherein slugs (I) and (II) are injected batchwiseinto the formation.
 9. The process of claim 1 wherein water is addedcontinuously to produce slugs (I) and (II) of variable water contentsduring their injection into the formation.
 10. The process of claim 1wherein slugs (I) and (II) have a lower hydrocarbon to surfactant ratiothan said primary oil-external solution.
 11. The process of claim 1wherein the ratio of the total weight of inorganic salts to the weightof total water in slugs (I) and (II) is lower than in the oil-externalsolution.
 12. The process of claim 1 wherein the cosurfactant tosurfactant ratio in slugs (I) and (II) is greater than that in theprimary oil-external solution.
 13. In the injection of a water streamthrough an injection well into a porous subsurface formation containinga water immiscible phase which is resistant to displacement by waterflow which substantially restricts the flow of water through theformation and wherein the inplace water in said formation has a saltcontent in excess of about 5,000 ppm, the improvement which comprises:removing said water immiscible phase and thereby inCreasing theinjectivity of said formation to water with a resulting substantialincrease in the rate of water injection by first injecting a slug offresh water into said formation; thereafter injecting into the formationa primary oil-external micellar solution in an amount sufficient todisplace said water immiscible phase out into said formation; followingthe injection of said oil-external solution with a micellar slug (I) ofsubstantially increased water content over said oil-external solution,thereby displacing said water immiscible phase by said oil-externalsolution into the region about the well bore; thereafter injecting waterinto said well and into said formation in sufficient amount to displacesaid oil-external micellar solution and said micellar slug (I) fartherout into said formation and saturating said region in turn with water;and continuing said water injection at a substantially increased rate.